It is known that the first objective of distributed feedback is to maximize the elimination of the laser's secondary modes. A second objective is generally to reduce the mirror losses, that is, the losses due to the finite length of the laser cavity.
Currently, there are two main technologies for producing distributed feedback (DFB) lasers.
These two known technologies are on the one hand the use of buried gratings, in order to produce an index modulation, and on the other hand the use of surface metallic gratings, in order to produce a gain modulation.
The so-called “buried grating” technology consists in etching a periodic grating of notched thickness after growing the active area of the semiconductor laser. Then, the top layer of the laser waveguide is produced by a new step of growth on the grating. This technology introduces an index coupling because the wave being propagated in the active area perceives a modulation of the effective index of the cavity which changes with the thickness. It makes it possible to obtain a strong distributed feedback without degrading the laser threshold, that is, without introducing additional loss.
The main drawback of this technology is that it entails interrupting the growth between the active area and the waveguide in order to etch the grating then to carry out a new epitaxial growth step. This step is technologically difficult to perform, notably because an epitaxy must be performed on the grating, therefore on a surface that is not perfectly flat. Furthermore, the distributed feedback performance levels are very sensitive to the profile of the etching of the grating in the semiconductor layer close to the active area; any variation of the etching depth directly influences the grating's coupling coefficient. This makes the feedback difficult to control.
The second known technology for producing distributed feedback lasers is the “metallized surface grating” technology. This technology makes it possible to produce a gain coupling (or loss coupling). The object here is to grow all the layers of the laser: active area, waveguides and contact layers. A pattern is then etched in the top waveguide, then metallized. Compared to the buried grating technology, this technology presents the advantage that the steps introducing the distributed feedback, in particular the etching of the pattern in the top waveguide, are carried out after the growth of all the layers. It is therefore simpler to implement. On the other hand, its main defect is that the distributed feedback is obtained by loss modulation. This introduces additional losses, increasing the laser threshold and therefore degrading the laser's performance. Furthermore, the distributed feedback is, as with the buried grating technology, difficult to control because it is very sensitive to the accuracy of the etched pattern. Finally, the distributed feedback that is obtained is also linked to the optical losses introduced by the metal and the constituent materials of the waveguide, which constitutes another parameter that is difficult to control.
To sum up, the existing technologies for developing distributed feedback lasers are generally difficult to implement and to control and can degrade the laser's performance.
It should be noted, however, that preference is ideally given to index modulation lasers which, despite their being difficult to implement, present lower losses than loss modulation lasers.